Multipath audio stimulation using audio compressors

ABSTRACT

An apparatus for delivering audio signals to a user includes an air-conduction transducer and a bone-conduction transducer. The air-conduction transducer is configured to convert a component of the audio signals to air vibrations detectable by an ear of the user. The bone-conduction transducer is configured to convert another component of the audio signals to vibrations in a cranial bone of the user via direct contact with the user. The apparatus employs one or more filters to separate input audio signals to produce a high-frequency component, a mid-frequency component, and a low-frequency component. The mid- and low-frequency components are processed using compressors to reduce the dynamic range of the components, and then combined to produce a combined component. The combined component is delivered to the user through the bone-conduction transducer, and the high-frequency component is delivered to the user through the air-conduction transducer.

PRIORITY INFORMATION

This application claims priority to U.S. provisional patent applicationSer. No. 62/511,844, titled “Multipath Audio Stimulation Using AudioCompressors,” filed May 26, 2017, which is hereby incorporated byreference in its entirety as though fully and completely set forthherein.

BACKGROUND Technical Field

This disclosure relates generally to audio communication systems thatdeliver audio signals to a user. More specifically, the disclosuredescribes audio communication systems that employ different modes ofaudio signal transduction to provide the audio signals to the user,including an air-conduction transducer and a bone-conduction transducer.

Description of the Related Art

Bone-conduction headphones allow a user to hear sounds through vibrationof the bones in the user's cranium. Bone-conduction headphones aredifferent from air-conduction headphones, which convert sound signals toair vibrations that are then detected by the human ear. With boneconduction, sound signals are transmitted directly to the user's body,via direct contact with the user. Different types of bone-conductionheadphones can operate by contacting different portions of the user'shead and transmitting sound through different portions of the user'scranium. For example, a set of bone-conduction headphones may bedesigned to vibrate the temporal bones on the sides of the user's face,either in front of or behind the user's ear. Other bone-conductionheadphones may be designed to transmit sound through other parts of thecranium, such as the sphenoid bone, the jaw bone, or the nasal bone.

Bone-conduction headphones provide a number of advantages over airconduction headphones. For example, because bone-conduction headphonesdo not obstruct the air in the user's ear canal, bone-conductionheadphones allow the user to continue to hear external sound along withthe transmitted sounds. Bone-conduction headphones are also useful forusers whose normal auditory pathway is impaired or damaged by bypassingthe impaired portions of the auditory anatomy. Bone-conductionheadphones are also useful in environments where air conduction is notpossible, such as underwater environments.

Despite their usefulness, however, bone-conduction headphones alsopresent a number of drawbacks. First, bone conduction does not work wellfor high-frequency sounds. The human audible range generally ranges from20 to 20,000 Hertz. However, the effectiveness of bone conductiondecreases significantly at frequencies higher than approximately 4,000Hz. Moreover, because bone conduction works by direct contact with thehead, bone-conduction headphones may cause a tactile sensation on theuser's skin. This tactile sensation is especially noticeable when thebone-conduction headphones are operating at a high intensity, andresults in to a tickling sensation to the user, which can range up toannoying. These problems generally limit the usability ofbone-conduction headphones for ordinary consumers.

SUMMARY OF EMBODIMENTS

An apparatus for delivering audio signals to a user is disclosed herein.That apparatus may receive or generate the audio signals to be deliveredto the user. The apparatus may comprise a headphone, a headset, anearpiece, a hearing aid, or the like that includes an air-conductiontransducer and a bone-conduction transducer. The air-conductiontransducer may be configured to convert a component of the audio signalsto air vibrations detectable by an ear of the user. The bone-conductiontransducer may be configured to convert another component of the audiosignals to vibrations in a cranial bone of the user via direct contactwith the user. Thus, the apparatus is capable of delivering the audiosignals to the user via both transducers. In some embodiments, theair-conduction transducer may be designed such that it can generatesounds into an ear cavity of the user without substantially blocking theear cavity from other sounds. Thus, such embodiments retain the benefitof standalone bone-conduction headphones that permits external sound tobe received by the user.

In some embodiments, the apparatus may employ one or more filters toseparate input audio signals into different frequency ranges. Inembodiments, the ranges may include a first frequency range and a secondfrequency range. The first frequency range, which may be ahigh-frequency range, may be delivered to the user through theair-conduction transducer. The second frequency range, which may be alow-frequency range, may be delivered to the user through thebone-conduction transducer.

In some embodiments, audio processing techniques may be employed toreduce the dynamic range of the components. For example, compressors maybe used to reduce the output intensity of the components relative to theinput intensity. The compression may be performed on components of theaudio signals that are delivered through the bone-conduction transducer,so as to reduce the tickling sensation to the user at high intensitylevels. In some embodiments, the compression may be tuned to balance thepsychophysical response in the user produced by the differenttransducers. Thus, components of the audio signal delivered via theair-conduction transducer and the bone-conduction transducer may beperceived by the user to be at approximately the same degree ofloudness.

In some embodiments, the bone-conduction component of the audio signalsmay be subdivided into smaller frequency components and processeddifferently. Thus, for example, different subcomponents of thebone-conduction component may be compressed according to differentinput-output transfer functions. By using multiple frequencysubcomponents, the processing of the bone-conduction component of theaudio signals may be configured more flexibly. The frequency componentsmay then be recombined and transmitted to the bone-conductiontransducer. In some embodiments, parameters of the processing, includingthe filter ranges and the compression settings, may be configurable bythe user via a graphical user interface or user controls on theapparatus. These and other features of the inventive apparatus aredescribed in further detail below, in connection with the Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a headset that includes an air-conductiontransducer and a bone-conduction transducer, according to someembodiments.

FIG. 2 illustrates components used to process audio signals deliveredvia an air-conduction transducer and a bone-conduction transducer,according to some embodiments.

FIGS. 3A, 3B, and 3C illustrate example input-output transfer functionsfor different audio signal components generated for an air-conductiontransducer and a bone-conduction transducer, according to someembodiments.

FIGS. 4A and 4B illustrate two example embodiments of an apparatus thatdelivers audio signals via an air-conduction transducer and abone-conduction transducer, according to some embodiments.

FIG. 5 is a flow diagram that illustrates operations performed by anapparatus that includes an air-conduction transducer and abone-conduction transducer, according to some embodiments.

FIG. 6 illustrates a computer system that may be configured toimplement, operate, or configure portions of an apparatus that includesan air-conduction transducer and a bone-conduction transducer, accordingto some embodiments.

DETAILED DESCRIPTION

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps. Consider aclaim that recites: “An apparatus comprising one or more processor units. . . .” Such a claim does not foreclose the apparatus from includingadditional components (e.g., a network interface unit, graphicscircuitry, etc.).

“Configured To.” Various units, circuits, or other components may bedescribed or claimed as “configured to” perform a task or tasks. In suchcontexts, “configured to” is used to connote structure by indicatingthat the units/circuits/components include structure (e.g., circuitry)that performs those task or tasks during operation. As such, theunit/circuit/component can be said to be configured to perform the taskeven when the specified unit/circuit/component is not currentlyoperational (e.g., is not on). The units/circuits/components used withthe “configured to” language include hardware—for example, circuits,memory storing program instructions executable to implement theoperation, etc. Reciting that a unit/circuit/component is “configuredto” perform one or more tasks is expressly intended not to invoke 35U.S.C. § 112, sixth paragraph, for that unit/circuit/component.Additionally, “configured to” can include generic structure (e.g.,generic circuitry) that is manipulated by software and/or firmware(e.g., an FPGA or a general-purpose processor executing software) tooperate in manner that is capable of performing the task(s) at issue.“Configure to” may also include adapting a manufacturing process (e.g.,a semiconductor fabrication facility) to fabricate devices (e.g.,integrated circuits) that are adapted to implement or perform one ormore tasks.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, a buffer circuitmay be described herein as performing write operations for “first” and“second” values. The terms “first” and “second” do not necessarily implythat the first value must be written before the second value.

“Based On.” As used herein, this term is used to describe one or morefactors that affect a determination. This term does not forecloseadditional factors that may affect a determination. That is, adetermination may be solely based on those factors or based, at least inpart, on those factors. Consider the phrase “determine A based on B.”While in this case, B is a factor that affects the determination of A,such a phrase does not foreclose the determination of A from also beingbased on C. In other instances, A may be determined based solely on B.

FIGS. 1A and 1B illustrate a headset that includes an air-conductiontransducer and a bone-conduction transducer, according to someembodiments. As shown, the apparatus 100 may comprise a headset that isdesigned to be worn on the head of a user. However, the inventiveconcepts disclosed herein may be implemented in articles or devicesother than a monolithic headset. For example, in some embodiments, theair-conduction transducer and the bone-conduction transducer may be twoseparate pieces. In some cases, the bone-conduction transducer maycomprise a subdermal implant. The headset 100 may include othercomponents not shown in the Figures. For example, in some embodiments,the headset 100 may comprise a computer capable of generating contentand/or communicating with other computers via a network interface. Asanother example, the headset 100 may comprise a peripheral device thatis controllable by another computing device proximal to the user, suchas the user's smartphone. In some cases, the headset 100 may include apair of viewing glasses capable of displaying a video to the user oraugmenting the visual perceptions of the user. The headset 100 may alsoinclude user input controls 130 that allows the user to interact withthe headset. For example, the headset may include a microphone that iscapable of capturing voice commands spoken by the user. In some cases,the headset 100 may include sensors capable of capturing physiologicaldata from the user, such as the user's heart rate. The headset 100 mayinclude a power source, such as a battery, or an interface to anexternal power source.

As shown, the headset 100 includes an interface to connect to andreceive audio signals from an audio signal source 110. The audio signalsource 110 may comprise any type of source or device capable ofgenerating or transmitting audio signal to be delivered to the user.Examples of such audio signal sources may include for examplesmartphones, tablets, computers, music players such as CD or cassetteplayers, radios, televisions, musical instruments, hearing aids,microphones, speakers, and the like. In some embodiments, the audiosignal source 110 may be part of the headset 100. For example, theheadset 100 may comprise a wearable computing device such as a pair ofaugmented reality or virtual reality glasses capable of generatingaudio. Moreover, although the FIG. 1A depicts the audio signal source tobe connected to the headset 100 via a wire, in other embodiments theconnection may be wireless. For example, the headset 100 may beconnected to the audio signal source 110 to receive audio signals via atype of wireless protocol. Such protocols may include for exampleBluetooth, WiFi, a cellular protocol such as CDMA or GSM, or any othertype of commercial or proprietary wireless protocol.

The headset 100 includes an air-conduction transducer 120 and abone-conduction transducer 125. The air-conduction transducer isdesigned to convert audio signals to vibrations in the air that aredetectable by the user's ear 122. Specifically, air vibrations in theear canal 130 vibrates the ear drum 132, which in turn cause vibrationsin the cavity of the user's middle ear, ultimately causing vibrations inthe bony chamber of the cochlea 134. The inside of cochlea 134 is linewith sensory hair cells that are connected to the user's auditory cortex136.

The headset 100 may include two air-conduction transducers 120, one foreach respective ear 122 of the user. The air-conduction transducer 120may comprise a loudspeaker that is placed in or in proximity to theuser's ear canal 130. The air-conduction transducer 120 may bepositioned to direct sound into the user's ear canal 130. In someembodiments, air-conduction transducers 120 may be configured such thatthey do not substantially block the opening to the ear canal 130, sothat the ear can continue to receive external sound from the user'ssurroundings. For example, the air-conduction transducer 120 may besuspended via an attachment to the headset 100 or an earpiece, whichpositions the air-conduction transducer 120 in such a way that avoidssubstantially blocking the user's ear from external sound. In otherexamples, the air-conduction transducer 120 may include an opening thatallows external sound to reach the user's ear canal 130. The positioningof the air-conduction transducer 120 or the opening thereon may beconfigurable by the user to accommodate the user's particular anatomy orpreferences. In some cases, the headset 100 may utilize to hearing aidtechnology to capture external sound and amplify these sounds to theuser via the air-conduction transducers 120. The amplification ofexternal sound may be adjustable by the user via controls on the headset100.

Unlike the air conduction, bone conduction delivers sound to the user'sinner ear via vibrations in the user's cranial bones. For example, boneconduction is a primary manner in which one hears one's own voice. Asshown in the Figures, a bone-conduction transducer 124 is included toconvert audio signals from the audio signal source 110 to vibrations inthe human body. In particular, the bone-conduction transducer 124 isdesigned generate vibrations in a cranial bone 126 of the user. Indifferent situations, the bone-conduction transducer 124 may be placedagainst with the user's body, as shown, or implanted inside the body.The vibrations in the cranial bone 126 may be detected by the bonycochlea chamber 134 of the inner ear, which is communicated to theauditory cortex 136, resulting in an auditory sensation to the user. Thebone conduction pathway thus bypasses the normal auditory pathway of theouter and middle ear.

The headset 100 may include multiple bone-conduction transducers 124.For example, the headset 100 may include two bone-conduction transducers124, one for each side of the user's face. In other examples, more thantwo bone-conduction transducers 124 may be used for different bones. Forexample, one embodiment of the headset 100 may employ four separatebone-conduction transducers 124 placed on four different points on theuser's head. Bone-conduction transducers may be placed any location onthe user's head. The locations are preferably close and fixed relativeto the underlying cranial bones and the cochlea chamber in the user'shead. For example, one location for the bone-conduction transducer maythe mastoid portion of the temporal bone of the human skull, which islocated just behind the ear. This location is proximal to the mastoidprocess, which is located just behind the middle ear. Research has shownthat other locations on the head with high bone-conduction sensitivityinclude the forehead, the vertex or top of the head, and variouslocations on the side of the face, in particular near the temple or theear. In some embodiments of the headset 100, the location may beselectable by the user. For example, the headset 100 may permit anadjustable number of bone-conduction transducers to be added, eachdesigned to contact a particular portion of the head. The headset 100may deliver the audio signals, or different components of the audiosignals, to each of the bone-conduction transducers and/or audioconduction transducers approximately at the same time, to provide theuser a multimode listening experience. In some embodiments, thedifferent components may be processed (e.g., by shifting the relativetime of the components), so as to provide a stereophonic sound to theuser. The different components of the audio signal provided to thetransducers may be configured by the user to improve balance andsynchronization among the transducers.

In some embodiments, the headset 100 may further include amicrocontroller 140. The microcontroller 140 may comprise a computerimplemented on an integrated circuit. The microcontroller 140 mayinclude software code configured to operate and configure variousaspects of the headset 100. For example, the microcontroller 140 mayimplemented a digital signal processor (DSP) to perform signalprocessing on received audio signals. In embodiments where the audiosignals are received in analog form, the microcontroller 140 may employan analog-to-digital converter (ADC) to convert the analog signals todigital form. The microcontroller 140 may then perform a number ofsignal processing operations on the digital signals. In someembodiments, the microcontroller 140 may implement a number of filtersto separate the audio signals into different frequency components. Thedifferent frequency components may be processed differently, before theyare recombined and/or forwarded to the various air or bone-conductiontransducers. In embodiments, the digital signals may be converted backto analog signals using a digital-to-analog converter (DAC) to drive thevarious transducers. In some embodiments, certain transducers on theheadset 100 may only receive some subset of frequency band components ofthe audio signal. For example, in some cases, higher frequencycomponents are forwarded to the air-conduction transducers 120, whilelower frequency components are forwarded to the bone-conductiontransducers 124.

The processing of the components of the audio signals may include anumber of operations that will be discussed in further detail inconnection with FIGS. 2 and 3. In particular embodiments, the dynamicrange of a component may be reduced using an audio compressor. That is,the audio compressor may reduce the volume of loud sounds in the audiosignals or amplify the soft sounds in the audio signals. Thiscompression of the audio signals may reduce the tickling sensation thatis generated by the bone-conduction transducers.

In some embodiments, different components of the audio signals may becompressed differently using different input-output transfer functions.Studies have shown that the tactile sensations produced bybone-conduction transducers are frequency-dependent. In particular,lower frequency signals may produce more significant tactile sensationsin a user than higher frequency signals. Accordingly, in someembodiments, a compressor for low frequency audio signal components maybe set to compress the dynamic range of those components moreaggressively than high frequency components. The individual compressorsettings may be set initially by the manufacturer of the headset 100,and then subsequently adjusted by the user using psychophysicalcalibration techniques. For example, the headset 100 or an accompanyingdevice may be programmed to perform a psychophysical test on the user bygenerating sounds at different frequencies and intensities to the user.The user may then provide feedback as to the amount of tactile sensationthat was experienced. Based on the feedback response, the headset 100may calibrate the settings for the compressors or the microcontroller140 to provide a more comfortable listening experience for the user.

Another signal processing operation that may be performed on the audiosignal components is equalization. In this operation, the intensity ofdifferent frequency components within the audio signal are adjusted.Such adjustments may be made for tone control or for aesthetic reasons,or technically by measurement. Equalization may be performed using anequalizer that is implemented separately or as part of the DSP processoron the microcontroller 140. Equalization and compression may both beused to achieve a better balance of the loudness of sounds generated bythe different transducers, producing a more comfortable listeningexperience to the listener.

To properly equalize the sounds from two different types of transducers,a psychophysical calibration may be performed to determine the user'sresponse to stimuli from both transducers. For example, in onecalibration method, two phase cancelling signals may be transmitted atdifferent volumes via the two conduction paths to the user. The userwould then provide feedback to report the point at which the two signalsare perceived to perfectly cancel one another. This point thusrepresents a loudness matching of the two types of transducers. Theresults of such psychophysical calibration may be used to define a setof equivalence loudness levels between the two types of transducers, interms of input energy. The equivalence loudness level may then be usedto equalize audio signals between the two types of transducers. Similarpsychophysical calibration techniques may be used to achieve a balancingbetween different bone-conduction transducers on the headset 100. Inparticular, some locations on the cranium may be more sensitive to soundconduction than other locations. Such differences may be accounted forthrough equalization.

In some embodiments, the headset 100 may include user controls 150 thatallow the user to dynamically adjust parameter settings of the DSPand/or the microcontroller 140. For example, user controls 150 mayinclude physical control elements such as switches, knobs, or dials. Thecontrol elements may perform functions such as powering on or off theheadset 100 or particular elements (e.g., transducers) on the headset100. The control elements may also allow the user to adjust operatingsetting of the headset 100, such as the volume of particulartransducers, the frequency range assigned the different transducers, andvarious other settings of the audio compressors and equalizers. In someembodiments, some of the user controls 150 may be accessible via asoftware user interface. For example, the headset 100 may be coupledwith an accompanying computing device, such as a smartphone, and anapplication on the smartphone may allow the user to configure varioussettings of the headset 100. As another example, the headset 100 mayinclude a visual display (e.g., in a pair of glasses) and a microphonecapable of receiving audio commands from the user. These features of theheadset 100 may be used to configure the headset's audio settings.

FIG. 2 illustrates components used to process audio signals deliveredvia an air-conduction transducer and a bone-conduction transducer,according to some embodiments. Some of the components of FIG. 2 may beimplemented as analog devices. Alternatively, the components may beimplemented as portions in a digital signal processor (DSP). In someembodiments, the DSP may be implemented as a part of the microcontroller140, as discussed in connection with FIG. 1.

As shown in the FIG. 2, input audio signal 202 is received. The inputaudio signal 202 may be received from an audio signal source 110, asdiscussed in connection with FIG. 1. The input audio signal 202 may thenbe processed filters 210, 212, and 214, to produce three differentfrequency band components of the input audio signal 202. Filter 214 maybe configured to produce a high-frequency component of the audio signal202, filter 212 a mid-frequency component, and filter 210 a lowfrequency component. The high-frequency component may be transmitted toa driver for the air-conduction transducer 120, while the mid- andlow-frequency components may be transmitted to a driver for thebone-conduction transducer 124. In embodiments, components that areprovided to the bone-conduction transducer are limited to frequencyranges below 4,000 Hz, as the effectiveness of bone conduction decreasessignificantly at frequencies higher than the threshold. In someembodiments, the bone-conduction transducer is used to transmit basscomponents, which are generally understood to include frequencies lowerthan 250 Hz. The audio signal provided to the bone-conduction transducermay be further divided into two or more subcomponents. For example, inthe Figure, the filters produce a low-frequency and a mid-frequencyfrequency component that are transmitted to the bone-conductiontransducer. In alternate embodiments, only one component or more thantwo subcomponents may be transmitted to the bone-conduction transducer.By dividing the bone-conduction transducer components into multiplesubcomponents, the subcomponents may be processed differently. Forexample, because the tactile sensation generated by the bone-conductiontransducer are more noticeable at lower frequencies, the apparatus mayemploy a more aggressive audio compressor with a flatter compressionslope to compress that component. The apparatus thus implements aflexible signal processing procedure, depending on the frequency of theaudio signal component.

The filters 210, 212, and 214 may be configured to allow a particularrange of frequencies of audio signal 202 to pass through, while at thesame time inhibiting other ranges of frequencies. The filter mayprogressively attenuate the audio signal 202 in frequencies outside ofthe pass band. In some cases, the filter may amplify the audio signal202 in the frequency range of the filter's pass band. The frequencyranges may be selected such that they fall within the transductioncapabilities of the transducer. The frequency ranges may also beselected so that they correspond roughly to the standard bass (30-250Hz), midrange (250-2000 Hz), and treble (2-16 kHz) ranges as generallyused in the sound equipment industry. In one preferred embodiment,filter 210 may be configured to produce a pass band of approximately 60Hertz and below, filter 212 may be configured to produce a pass band ofapproximately 60 to 250 Hertz, and filter 214 may be configured toproduce a pass band of approximately 250 Hertz and above. Differenttypes of audio content may have different dynamic frequency ranges,which may be assigned to appropriate transducers. For example, humanspeech typically falls within the frequency range of 85 Hz to 8 kHz.This particular range may be isolated using one or more filters so thatthese audio signal components may be processed separately. As would beunderstood by a person of ordinary skill in the art, however, differentfrequency ranges may be used in other embodiments.

The filters may be implemented or configured as one or more active audiocrossovers. In a preferred embodiment, filters 210 and 214 may beimplemented using or configured as a 4th order Butterworth squaredfilter or a 4th order Linkwitz-Riley crossover (LR4), and filter 212 maybe implemented using or configured as two 4th order Butterworth squaredfilters or two 4th order Linkwitz-Riley crossover (2×LR4). These filtersare designed to achieve a flat frequency response in the pass band andsufficiently steep roll off in the stop band. However, in otherembodiments, different types of filters may be used and additionalnumbers of frequency band components may be produced.

In some embodiments, the different frequency components produced by thefilters may be further processed by three respective equalizers 220,222, and 224. The equalizers 220, 222, and 224 may be implemented usinganalog devices or programmed using one or more digital signalprocessors, as discussed in connection with FIG. 1. During equalization,the intensity of different frequency components within the audio signalmay be adjusted. Such adjustments may be used for controlling the toneof the input audio signals 202. In a setting where one or morebone-conduction transducers 124 are used, equalizers 220 and 222 may beused to attenuate components of the audio signals so as to reduce anytactile sensation caused by those transducers. In different embodiments,the equalizers 220, 222, and 224 may be configured to process the audiosignal at different points in the signal processing procedure.Equalization may take place before or after a compression of the dynamicrange of the audio signal. In some embodiments, the equalizers 220, 222,and 224 may be configured as a group to enhance the overall balanceamong all transducers on the apparatus. The equalizers 220, 222, and 224may be configurable via user controls 150 or a different user interface,which may be accessible via the headset 100 or some other device, asdiscussed in connection with FIG. 1.

In some embodiments, the different frequency components may be processby audio compressors, such as compressors 230, 232, and 234 shown inFIG. 2. The compressors 230, 232, and 234 may be implemented usinganalog devices or programmed using one or more digital signalprocessors, as discussed in connection with FIG. 1. During audiocompression, the dynamic range of the respective audio signal componentsproduced by the filters is reduced. The compressors may attenuate softersounds in the audio signal component and/or amplify louder sounds in theaudio component to narrow the intensity range of the component. Eachcompressor 230, 232, and 234 may compress its respective componentdifferent, according to a respective input-output transfer function. Theinput-output transfer function generally specifies the relationshipbetween the input power of an incoming audio signal and the output powerof the outgoing audio signal. The compression of the audio signalcomponents may take place any different points during the signalprocessing procedure. In some embodiments, audio compression may occurafter an equalization of the respective audio signal components. In someembodiments, the equalizer and the compressor may comprise a singlephysical circuit or logical unit in a DSP.

For audio signal components that are to be delivered via abone-conduction transducer 124, the compressor may attenuate the audiosignal to reduce the tactile sensation that may be generated by thetransducer. The individual compressor settings may be set initially bythe manufacturer of the apparatus, and then subsequently adjusted by theuser using psychophysical calibration techniques. For example, a headset100 or an accompanying device may be programmed to perform apsychophysical test on the user by generating sounds at differentfrequencies and intensities to the user. The user may then providefeedback as to the amount of tactile sensation that was experienced.Based on the feedback response, the headset 100 may determine particularsetting for the compressors 230 and 232, for example the compressionthreshold and/or the compression ratio of these compressors. Thesecompressors may be calibrated accordingly to provide a more comfortablelistening experience for the user. The compressors 230, 232, 234 may befurther configurable via user controls 150 or a different userinterface, which may be accessible via the headset 100 or some otherdevice, as discussed in connection with FIG. 1.

In some embodiments, different components of the audio signal componentsmay be recombined using a combiner. For example, as shown, thelow-frequency and mid-frequency components of the audio signal 202 maybe combined using combiner 240. The combiner 240 may be implementedusing analog devices or programmed using one or more digital signalprocessors, as discussed in connection with FIG. 1. In some alternativeembodiments, the combiner 240 may aggregate more than two frequencycomponents to produce the combined signal for the bone-conductiontransducer. In other alternative embodiments, the component of the audiosignal 202 provided to the air-conduction transducers may also beseparated into subcomponents for separate processing, and a secondcombiner may be used to recombine those subcomponents for theair-conduction transducer. In some embodiments, the combiner 240 mayperform further balancing of the incoming audio signals, such asamplitude or phase balancing.

FIGS. 3A, 3B, and 3C illustrate example input-output transfer functionsfor different audio signal components generated for an air-conductiontransducer and a bone-conduction transducer, according to someembodiments. These input-output transfer functions specify thecompression characteristics of the audio compressors that are used inthe apparatus, such as the headset 100 of FIG. 1. As illustrated, thelow-frequency input-output transfer function 310 may correspond to thecompressor 230 for the low-frequency component in FIG. 2, themid-frequency input-output transfer function 350 may correspond to thecompressor 232 for the mid-frequency component, and the high-frequencyinput-output transfer function 360 may correspond to the compressor 234for the high-frequency component. In the Figures, for example FIG. 3A,the horizontal axis 320 represents the level of input power of thesignal provided to the compressor. The vertical axis 330 represents thelevel of output power of the signal generated by the compressor. Forexample, the horizontal axis and 320 and vertical axis 330 may expressedin terms of decibels. The dotted line 340 in FIG. 3A represents theinput-output transfer function where no audio compression is applied tothe signal, where the output level of the signal simply equals to theinput level.

In FIG. 3A, the low-frequency input-output transfer function 310 isrepresented by the solid sloped line in the graph. Transfer function 310may specify a compression threshold 312 and a compression ratio 314. Thecompression threshold 312 may specify an input level above which thecompressor begins to modify the incoming signal's intensity level. Insome embodiments where an expansion of the dynamic range is employed, asis the case for transfer function 310, the threshold 312 may specify theinput level below which expansion is applied. Expansions of the dynamicrange may be used for some range of low frequencies that are provided tothe bone-conduction transducer at very low power levels, because humansensitivity for low-frequency sounds are generally lower than forhigh-frequency sounds, and at low intensity levels, the ticklingsensations from the bone-conduction transducer may not be significant.In some cases, the threshold for an expansion and reduction of thedynamic range may be the same value. For example, in the transferfunction 310, the threshold point 312 serves both an ending point for adynamic range expansion for frequencies below that point, and also abeginning point for a dynamic range reduction for frequencies above thatpoint. In some cases, multiple levels of audio compression may be usedfor different ranges of input intensity levels, and accordingly, thetransfer function may include multiple threshold points.

The compression ratio 314 represents the amount of gain reduction orincrease that is produced by the compressor, given some amount of changein the intensity level of the input signal. For example, a compressionratio of 4:1 means that for every four decibels of increase that is seenin the input audio signal, the compressor reduces the intensity of theoutput signal by one decibel. In embodiments where both an expansion anda reduction of the dynamic range are implemented by the compressor, thecompressor may employ two different respective compression ratios. Insome embodiments, the compressor may implement multiple compressionratios for multiple ranges of input intensity levels, using multiplethreshold points.

FIG. 3B illustrates an exemplary input-output transfer function 350 of amid-frequency component of the audio signal. In the transfer function350, only a reduction of the dynamic range is employed. The compressionbegins at threshold 352, and at a compression ratio of 354. In someembodiments, the compression ratio 354 of a mid-frequency compressor maybe lower than the compression ratio of a lower frequency compressor, forexample the compression ratio 314. The tactile sensation from boneconduction is greater at lower frequencies. Accordingly, lower frequencycomponents of the audio signal may need to be compressed moreaggressively than higher frequency components. Moreover, the threshold352 of a mid-frequency compressor may be set lower than the threshold312 of a lower frequency compressor. This is because the tactilesensations from bone conduction may be felt at lower power levels forlower frequency sounds. In alternative embodiments where the boneconduction component of the audio signal includes more than twosubcomponents, each subcomponent may be compressed with a differentrespective compressor, which progressively increases the compressionthreshold and decreases the compression ratio for higher frequencycomponents.

The apparatus may allow the user to individually configure thecompressor settings for the multiple compressors to fine tune the devicefor listening comfort. The settings may be adjusted by for example theuser controls 150 as discussed in connection with FIG. 1, or a softwareuser interface accessible via the headset 100 or another computingdevice. In addition to the compressor threshold(s) and or compressorratio(s), other settings of the compressor may be configurable by theuser. For example, some embodiments of the apparatus may allow the userto adjust the attack and release times of the compressor, which relateto how quickly the compression begins after a triggering signal isdetected. In some embodiments, the user may be able to adjust thesoftness of the “knee” that occurs at the threshold point. A soft kneeat the threshold point means a gradual increase or decrease of thecompression ratio around the threshold point. A soft knee avoids theabrupt effects of the compressor when the input level of the audiosignal transitions between two compression ratios. In some embodiments,the compressors of the headset 100 employ an attack time of onemillisecond or less, and a release time of 50 milliseconds or less. Inpreferred embodiments, the compressors employ soft knees at thecompression thresholds.

FIG. 3C illustrates another exemplary input-output transfer function 360for a high-frequency component of the audio signal. Transfer function360 may correspond for example compressor 234 for the air-conductiontransducer 120, as discussed in FIG. 2. As illustrated, transferfunction 360 includes a threshold 362 and a compression ratio 364. Inembodiments, the threshold 362 may be higher than the thresholds oflower frequency compressors, such as thresholds 312 and 352 in FIGS. 3Aand 3B. In embodiments, the compression ratio 364 may be lower than thecompression ratios of lower frequency compressors, such as ratios 314and 354 in FIGS. 3A and 3B. In some embodiments of the apparatus, theair-conduction transducer component(s) may not be compressed using anyaudio compressors. Only the components provided to the bone-conductiontransducer 124 may be compressed.

FIGS. 4A and 4B illustrate two example embodiments of an apparatus thatdelivers audio signals via an air-conduction transducer and abone-conduction transducer, according to some embodiments. FIG. 4Adepicts an exemplary embodiment where at least a portion of the signalprocessing components 418 reside in the headset 432 that includes theair-conduction transducer 420 and the bone-conduction transducer 422. Inthis embodiment, the signal source 412 may be a separate device 430 fromthe headset 432. The device 430 and the headset 432 may communicate viarespective interfaces 414 and 416. For example, the device 430 maycomprise a smartphone, a music player, or a computer. The interfaces 414and 416 may comprise the output and input ports of a physical wire, suchas a speaker, telephone, or data cable, or a wireless connection. Thewireless connection may comprise a data connection capable oftransmitting digital sound signals, such as a Bluetooth, WiFi, orcellular connection. In the embodiment of FIG. 4A, device 430 may not beadapted for generating audio signals that are adapted for air andbone-conduction transducers 420 and 422. Rather, the headset 432 mayaccept the input audio signals, perform signal processing on the signalsincluding splitting the signals into respective air and bone conductioncomponents, and then delivering the respective signals to the user viathe transducers. In this embodiment, the headset 432 may includespecialized DSP hardware and/or software that are adapted to workingwith the air and bone-conduction transducers 420 and 422 on the headset.

FIG. 4B depicts another exemplary embodiment where at least a portion ofthe signal processing components 444 reside in the device 450 thatincludes the signal source 442. In this embodiment, the interfaces 446and 448 may represent the output and input ports between the device 460and the headset 462. The interfaces 446 and 448 may implement a specialchannelized interface that specify different channels for theair-conduction transducer 450 and bone-conduction transducer 452 on theheadset 462. In some embodiments, the interface 446 and 448 may be anexisting interface for channelized audio signals, such as the connectioninterface for an audio mixer or a type of surround sound system. Inother embodiments, the interface 446 and 448 may implement a generaldata interface, such as the IP protocol, that implements a specializedhigh-level transmission protocol for the headset 462. In the embodimentof FIG. 4B, the device 460 may use the signal processing components 444to generate component audio signals that are adapted to be delivered bythe headset 462. For example, the device 460 may be a smartphone, acomputer, or gaming console that includes specialized driver software tooperate with the headset 462.

In both embodiments in FIGS. 4A and 4B, the signal processing components418 and 444 may be configurable by the user. For example, where in thesignal processing occurs on the headset 432, the headset 432 may beconfigurable by an accompanying device running configuration software,such as a computer or a smartphone. In the case where the signalprocessing is performed on the device 460, the configuration softwaremay be included as part of the driver software on the device 460. Theconfiguration software may permit the user to adjust various signalcontrol settings for the signal processing components 418 or 444. Forexample, the configuration software may allow the user to adjustsettings such as the equalization profile for the different equalizers.The configuration software may also allow the user to adjust thesettings for the compressors, such as the compression ratio andthreshold, and attack and release times. In some cases, theconfiguration software may adjust the settings for transducers onopposite sides of the head differently. For example, a user who is lesssensitive to bone conduction on the left side of her face may use moreaggressive compressor for the bone-conduction transducer on the leftside.

In some embodiments, the headset settings may be specific to aparticular user or device. For example, the signal source device andheadset may be configured to recognize one another, via a setupprotocol, when they are first connected. After recognition, either thedevice or the headset may apply a set of parameters that were previouslysaved for signal processing components. For example, a headset mayrecognize when it is connected to a particular smartphone, which isassociated with a particular set of audio parameters. The headset maythen retrieve or receive these parameters, and apply them to the signalprocessing components. The parameters may be saved on the device, theheadset, or on a remote server maintaining an account for the user.

In other embodiments, audio parameter settings may be associated withtypes of content. For example, different equalizer and/or compressorsettings may be associated with phone calls, as opposed to music.Because phone calls do not generally include sounds at the higher end ofthe audible spectrum that occur in music, the compressors and equalizersmay be configured differently to dispense with any processing performedto enhance the balance between low- and high-frequency audio signals.Particular sets of settings may also be associated with particularcontent, such as particular radio stations or songs. These settings maybe user configuration and stored in a configuration data repository withthe content. The settings may be applied when the content is played.

FIG. 5 is a flow diagram that illustrates operations performed by anapparatus that includes an air-conduction transducer and abone-conduction transducer, according to some embodiments. Process 500begins at operation 502, where audio signals are received from an audiosignal source. Operation 502 may be performed by a headset, for exampleheadset 100 as discussed in connection with FIG. 1. Alternatively,operation 502 may be performed by a device separate from the headset,for example device 460 as discussed in connection with FIG. 4B.

At operation 504, the audio signals are filtered to produce ahigh-frequency component, a mid-frequency component, and a low-frequencycomponent. In some embodiments, only two components may be generatedfrom the filters, one for the air-conduction transducer and one for thebone condition transducer. In some other embodiments, more than threecomponents may be produced as the result of the filtering. The filtersmay be parts of an audio crossover network, and may be implementedeither as analog circuits or as part of a DSP. In one preferredembodiment, the high- and low-frequency components may be produced by arespective 4th order Butterworth squared filter or a 4th orderLinkwitz-Riley crossover (LR4), and the mid-frequency component may beproduced by two 4th order Butterworth squared filters or two 4th orderLinkwitz-Riley crossover (2×LR4). In one preferred embodiment, filtersmay be configured to produce a low-frequency component of approximately60 Hertz and below, a mid-frequency component of approximately 60 to 250Hertz, and high-frequency component of approximately 250 Hertz andabove.

At operation 506, the high-, mid-, and low-frequency components areequalized. The equalization may be performed by separate equalizersimplemented as analog circuits or as part of a DSP. The equalization maybe performed by the headset 100, or a separate computing device such asdevice 460. The equalizers may comprise three separate equalizers or asingle equalizer configured to balance the three audio signal componentsglobally. The equalization may take place before or after a compressionof the dynamic range of the audio signal components. In someembodiments, the equalization may be performed by adjusting theamplitude of the various audio signal components to achieve a balancebetween the air-conduction transducer and bone-conduction transducer,according to an equivalent loudness function of the two transducers thatis determined using a psychophysical calibration by the user.

At operation 508, the high-frequency component of the audio signal istransmitted to an air-conduction transducer. The air-conductiontransducer may be air-conduction transducer 120 as discussed inconnection with FIG. 1. The high-frequency component may be transmittedto two air-conduction transducers attached to a headset, one for eachear of the user. The air-conduction transducers may be configured suchthat they do not substantially block the user's ear canal, thereby allowthe user to hear external noises from the surrounding environment. Insome cases, the air-conduction transducer may be shaped with an openingto allow external noises to reach the user's ear canal.

At operations 510 and 512, the dynamic ranges of the mid- andlow-frequency components are compressed based on a first and a secondinput-output transfer function, respectively. The compression may beperformed by two audio compressors, which may be implemented as analogcomponents or as part of a DSP. The compression may be performed by theheadset 100, or a separate computing device such as device 460.

The compression may be performed to balance the loudness of thebone-conduction transducers with the other transducers on the headset.The compression may also be performed to reduce the tactile sensationgenerated by the bone-conduction transducers, which may be prominent atlow-frequency ranges and at high intensity levels. The degree ofcompression may be calibrated according to psychophysical testingperformed by the headset's manufacturer or user. The compression maytake place before or after the equalization the audio signal components.In some embodiments, the compression ratio employed on the low-frequencycomponent is higher than the compression ratio employed on themid-frequency component. In some embodiments, the compression thresholdof the low-frequency compressor is lower than the compression thresholdof the mid-frequency compressor. In some embodiments, the compressorsmay be configured to increase the amplitude of the input signal at lowintensity levels. In some embodiments, the compressors may be configuredwith an attack time of one millisecond or less, and a release time of 50milliseconds or less. In preferred embodiments, the compressors employsoft knees at the compression thresholds.

At operation 514, the mid- and low-frequency components of the audiosignal are combined to produce a combined component. Operation 514 maybe performed by a combiner such as combiner 240 as discussed inconnection with FIG. 2. The combiner 240 may be implemented as analogcircuits or as part of a DSP. In some embodiments, the combiner 240 mayaggregate more than two frequency components to produce the combinedsignal for the bone-conduction transducer. In other alternativeembodiments, multiple components of the audio signal may be combined bya second combiner to produce a combined signal for the air-conductiontransducer. In some embodiments, the combiner may perform furtherbalancing of the incoming audio signals, including amplitude or phasebalancing.

At operation 516, the combined component is transmitted to abone-conduction transducer. The bone-conduction transducer may bebone-conduction transducer 124, as discussed in connection with FIG. 1.In some embodiments, the combined component may be transmitted tomultiple bone-conduction transducers attached to a headset, which aredesigned to contact different portions of the user's head. Thebone-conduction transducers may be designed to convert audio vibrationsinto vibrations into different cranial bones of the user's body,including for example the portions of the temporal bone in front of andbehind the user's ear, the cheek bone, sphenoid bone, the jaw bone,and/or the nasal bone. As discussed, a headset may deliver the audiosignals, or different components of the audio signals, to each of thebone-conduction transducers and/or audio conduction transducers atapproximately the same time, to provide the user a multimode listeningexperience. In some embodiments, the different components may beprocessed (e.g., by shifting the relative time of the components), so asto provide a stereophonic sound to the user.

FIG. 6 illustrates an example computer system 600 that may be configuredto include or execute any or all of the embodiments described above. Forexample, the computer system 600 may implement microcontroller 140 onthe headset 100, an accompanying device of the headset 100, or device430 or 460 discussed in connection with FIGS. 4A and 4B. In differentembodiments, the computer system 600 may be any of various types ofdevices, including, but not limited to, a personal computer system,desktop computer, laptop, notebook, tablet, slate, pad, or netbookcomputer, cell phone, smartphone, PDA, portable media device, mainframecomputer system, handheld computer, workstation, network computer, acamera or video camera, a set top box, a mobile device, a consumerdevice, video game console, handheld video game device, applicationserver, storage device, a television, a video recording device, aperipheral device such as a switch, modem, router, or in general anytype of computing or electronic device.

Various embodiments of a sensor data-processing system as describedherein, may be executed in one or more computer systems 600, which mayinteract with various other devices. Note that any component, action, orfunctionality described above with respect to FIGS. 1 through 5 may beimplemented on one or more computers configured as computer system 600of FIG. 6, according to various embodiments. In the illustratedembodiment, computer system 600 includes one or more processors 610coupled to a system memory 620 via an input/output (I/O) interface 630.Computer system 600 further includes one or more network interfaces 640coupled to I/O interface 630, and one or more input/output devices,which can include one or more user interface (also referred to as “inputinterface”) devices. In some cases, it is contemplated that embodimentsmay be implemented using a single instance of computer system 600, whilein other embodiments multiple such systems, or multiple nodes making upcomputer system 600, may be configured to host different portions orinstances of embodiments. For example, in one embodiment some elementsmay be implemented via one or more nodes of computer system 600 that aredistinct from those nodes implementing other elements.

In various embodiments, computer system 600 may be a uniprocessor systemincluding one processor 610, or a multiprocessor system includingseveral processors 610 (e.g., two, four, eight, or another suitablenumber). Processors 610 may be any suitable processor capable ofexecuting instructions. For example, in various embodiments processors610 may be general-purpose or embedded processors implementing any of avariety of instruction set architectures (ISAs), such as the x86,PowerPC, SPARC, or MIPS ISAs, or any other suitable ISA. Inmultiprocessor systems, each of processors 610 may commonly, but notnecessarily, implement the same ISA.

System memory 620 may be configured to store program instructions, data,etc. accessible by processor 610. In various embodiments, system memory620 may be implemented using any suitable memory technology, such asstatic random access memory (SRAM), synchronous dynamic RAM (SDRAM),nonvolatile/Flash-type memory, or any other type of memory. In theillustrated embodiment, program instructions included in memory 620 maybe configured to implement some or all of an ANS, incorporating any ofthe functionality described above. Additionally, existing automotivecomponent control data of memory 620 may include any of the informationor data structures described above. In some embodiments, programinstructions and/or data may be received, sent or stored upon differenttypes of computer-accessible media or on similar media separate fromsystem memory 620 or computer system 600. While computer system 600 isdescribed as implementing the functionality of functional blocks ofprevious Figures, any of the functionality described herein may beimplemented via such a computer system.

In one embodiment, I/O interface 630 may be configured to coordinate I/Otraffic between processor 610, system memory 620, and any peripheraldevices in the device, including network interface 640 or otherperipheral interfaces, such as input/output devices 650. In someembodiments, I/O interface 630 may perform any necessary protocol,timing or other data transformations to convert data signals from onecomponent (e.g., system memory 620) into a format suitable for use byanother component (e.g., processor 610). In some embodiments, I/Ointerface 630 may include support for devices attached through varioustypes of peripheral buses, such as a variant of the Peripheral ComponentInterconnect (PCI) bus standard or the Universal Serial Bus (USB)standard, for example. In some embodiments, the function of I/Ointerface 630 may be split into two or more separate components, such asa north bridge and a south bridge, for example. Also, in someembodiments some or all of the functionality of I/O interface 630, suchas an interface to system memory 620, may be incorporated directly intoprocessor 610.

Network interface 640 may be configured to allow data to be exchangedbetween computer system 600 and other devices attached to a network 685(e.g., carrier or agent devices) or between nodes of computer system600. Network 685 may in various embodiments include one or more networksincluding but not limited to Local Area Networks (LANs) (e.g., anEthernet or corporate network), Wide Area Networks (WANs) (e.g., theInternet), wireless data networks, some other electronic data network,or some combination thereof. In various embodiments, network interface640 may support communication via wired or wireless general datanetworks, such as any suitable type of Ethernet network, for example;via telecommunications/telephony networks such as analog voice networksor digital fiber communications networks; via storage area networks suchas Fibre Channel SANs, or via any other suitable type of network and/orprotocol.

Input/output devices may, in some embodiments, include one or moredisplay terminals, keyboards, keypads, touchpads, scanning devices,voice or optical recognition devices, or any other devices suitable forentering or accessing data by one or more computer systems 600. Multipleinput/output devices may be present in computer system 600 or may bedistributed on various nodes of computer system 600. In someembodiments, similar input/output devices may be separate from computersystem 600 and may interact with one or more nodes of computer system600 through a wired or wireless connection, such as over networkinterface 640.

Memory 620 may include program instructions, which may beprocessor-executable to implement any element or action described above.In one embodiment, the program instructions may implement the methodsdescribed above. In other embodiments, different elements and data maybe included. Note that data may include any data or informationdescribed above.

Those skilled in the art will appreciate that computer system 600 ismerely illustrative and is not intended to limit the scope ofembodiments. In particular, the computer system and devices may includeany combination of hardware or software that can perform the indicatedfunctions, including computers, network devices, Internet appliances,PDAs, wireless phones, pagers, etc. Computer system 600 may also beconnected to other devices that are not illustrated, or instead mayoperate as a stand-alone system. In addition, the functionality providedby the illustrated components may in some embodiments be combined infewer components or distributed in additional components. Similarly, insome embodiments, the functionality of some of the illustratedcomponents may not be provided and/or other additional functionality maybe available.

Those skilled in the art will also appreciate that, while various itemsare illustrated as being stored in memory or on storage while beingused, these items or portions of them may be transferred between memoryand other storage devices for purposes of memory management and dataintegrity. Alternatively, in other embodiments some or all of thesoftware components may execute in memory on another device andcommunicate with the illustrated computer system via inter-computercommunication. Some or all of the system components or data structuresmay also be stored (e.g., as instructions or structured data) on acomputer-accessible medium or a portable article to be read by anappropriate drive, various examples of which are described above. Insome embodiments, instructions stored on a computer-accessible mediumseparate from computer system 600 may be transmitted to computer system600 via transmission media or signals such as electrical,electromagnetic, or digital signals, conveyed via a communication mediumsuch as a network and/or a wireless link. Various embodiments mayfurther include receiving, sending or storing instructions and/or dataimplemented in accordance with the foregoing description upon acomputer-accessible medium. Generally speaking, a computer-accessiblemedium may include a non-transitory, computer-readable storage medium ormemory medium such as magnetic or optical media, e.g., disk orDVD/CD-ROM, volatile or non-volatile media such as RAM (e.g. SDRAM, DDR,RDRAM, SRAM, etc.), ROM, etc. In some embodiments, a computer-accessiblemedium may include transmission media or signals such as electrical,electromagnetic, or digital signals, conveyed via a communication mediumsuch as network and/or a wireless link.

The methods described herein may be implemented in software, hardware,or a combination thereof, in different embodiments. In addition, theorder of the blocks of the methods may be changed, and various elementsmay be added, reordered, combined, omitted, modified, etc. Variousmodifications and changes may be made as would be obvious to a personskilled in the art having the benefit of this disclosure. The variousembodiments described herein are meant to be illustrative and notlimiting. Many variations, modifications, additions, and improvementsare possible. Accordingly, plural instances may be provided forcomponents described herein as a single instance. Boundaries betweenvarious components, operations and data stores are somewhat arbitrary,and particular operations are illustrated in the context of specificillustrative configurations. Other allocations of functionality areenvisioned and may fall within the scope of claims that follow. Finally,structures and functionality presented as discrete components in theexample configurations may be implemented as a combined structure orcomponent. These and other variations, modifications, additions, andimprovements may fall within the scope of embodiments as defined in theclaims that follow.

What is claimed is:
 1. An apparatus, comprising: one or more filtersconfigured to separate audio signals to produce a high-frequencycomponent and a low-frequency component; a low-frequency compressorconfigured to reduce a dynamic range of the low-frequency componentbased at least in part on an input-output transfer function; anair-conduction transducer configured to convert the high-frequencycomponent to air vibrations detectable by an ear of a user; and abone-conduction transducer configured to convert the low-frequencycomponent with the reduced dynamic range to vibrations in a cranial boneof the user via direct contact with the user, wherein said reduce thedynamic range of the low-frequency component based at least in part onthe input-output transfer function attenuates components of the audiosignal associated with tactile sensations.
 2. The apparatus of claim 1,wherein the air-conduction transducer and the bone-conduction transducerare portions of a headset, and the low-frequency compressor is locatedon the headset.
 3. The apparatus of claim 1, wherein the low-frequencycompressor is implemented using one or more digital signal processors(DSP).
 4. The apparatus of claim 1, wherein the air-conductiontransducer and the bone-conduction transducer are portions of a headset,and the low-frequency compressor is implemented on a device distinctfrom the headset, the device providing a hardware interface to theheadset.
 5. The apparatus of claim 1, wherein the air-conductiontransducer is configured to generate sounds into an ear cavity of theuser without substantially blocking the ear canal from other sounds. 6.The apparatus of claim 1, wherein the one or more filters are configuredto separate the audio signals to produce a mid-frequency component;wherein the apparatus further comprises: a mid-frequency compressorconfigured to reduce a dynamic range of the mid-frequency componentbased at least in part on another input-output transfer function,wherein the input-output transfer function of the mid-frequencycompressor implements a compression ratio that is lower than acompression ratio implemented by the input-output transfer function ofthe low-frequency compressor; and a combiner configured to combine themid-frequency component and the low-frequency component to produce acombined component; and wherein the bone-conduction transducer isconfigured to convert the combined component to vibrations in a cranialbone of the user.
 7. The apparatus of claim 6, wherein the input-outputtransfer function of the mid-frequency compressor implements a firstcompression threshold, the input-output transfer function of thelow-frequency compressor implements a second compression threshold, andthe first compression threshold is greater than the second compressionthreshold.
 8. The apparatus of claim 6, wherein the combiner combinesthree or more frequency band components of the audio signals, includingthe mid-frequency component and the low-frequency component, whereineach component of the three or more components is compressed by arespective compressor.
 9. The apparatus of claim 6, further comprising:a first equalizer configured to equalize the high-frequency component; asecond equalizer configured to equalize the mid-frequency component; anda third equalizer configured to equalize the low-frequency component.10. The apparatus of claim 6, wherein the one or more filters attenuatesfrequencies below approximately 250 Hertz in the audio signals toproduce the high-frequency component, frequencies above approximately250 Hertz and below approximately 60 Hertz in the audio signals toproduce the mid-frequency component, and frequencies above approximately60 Hertz in the audio signals to produce the low-frequency component.11. A method, comprising: receiving, by a device comprising abone-conduction transducer, audio signals from an audio signal source;filtering the audio signals to produce a high-frequency component and alow-frequency component of the audio signals; compressing a dynamicrange of the low-frequency component based at least in part on aninput-output transfer function configured to attenuate components of theaudio signal associated with tactile sensations; transmitting thehigh-frequency component to an air-conduction transducer; andtransmitting the low-frequency component to the bone-conductiontransducer.
 12. The method of claim 11, wherein transmitting thehigh-frequency component to the air-conduction transducer generatessounds into an ear cavity of a user without substantially blocking othersounds from the ear canal.
 13. The method of claim 11, whereincompressing the dynamic range of the low-frequency component isperformed using one or more digital signal processors (DSP).
 14. Themethod of claim 11, further comprising: prior to transmitting thehigh-frequency component to the air-conduction transducer, equalizingthe high-frequency component; and prior to transmitting thelow-frequency component to the bone-conduction transducer, equalizingthe low-frequency component.
 15. The method of claim 11, furthercomprising: wherein the filtering of the audio signals produces amid-frequency component of the audio signals; further comprising:compressing a dynamic range of the mid-frequency component based atleast in part on another input-output transfer function; combining themid-frequency component and the low-frequency component to produce acombined component; and wherein the low-frequency component to abone-conduction transducer comprises transmitting the combined componentto the bone-conduction transducer.
 16. The method of claim 15, where:the input-output transfer function used to compress the dynamic range ofthe mid-frequency component implements a first compression ratio, andthe input-output transfer function used to compress the dynamic range ofthe low-frequency component implements a second compression ratio, andthe first compression ratio is lower than the second compression ratio.17. The method of claim 15, where: the input-output transfer functionused to compress the dynamic range of the mid-frequency componentimplements a first compression threshold, and the input-output transferfunction used to compress the dynamic range of the low-frequencycomponent implements a second compression threshold, and the firstcompression threshold is greater than the second compression threshold.18. A non-transitory computer-accessible storage medium storing programinstructions that when executed on one or more processors cause the oneor more processors to: receive audio signals from an audio signalsource; filter the audio signals to produce a high-frequency component,a mid-frequency component, and a low-frequency component of the audiosignals; compress the dynamic range of the mid-frequency component basedat least in part on a first input-output transfer function; compress thedynamic range of the low-frequency component based at least in part on asecond input-output transfer function; combine the mid-frequencycomponent and the low-frequency component to produce a combinedcomponent; transmit the high-frequency component to an air-conductiontransducer; and transmit the combined component to a bone-conductiontransducer.
 19. The non-transitory computer-accessible storage medium ofclaim 18, wherein the program instructions when executed on the one ormore processors cause the one or more processors to: compress thedynamic range of the mid-frequency component based at least in part on afirst input-output transfer function using a first compression ratio;and compress the dynamic range of the low-frequency component based atleast in part on a second input-output transfer function using a secondcompression ratio that is greater than the first compression ratio. 20.The method of claim 11, further comprising: configuring, in response toinput from user controls of the device, one or more compressors thatperform said compressing.